This invention relates to a distributed feedback semiconductor laser (referred to as DFB laser) which oscillates at a stable single wavelength.
A DFB semiconductor laser with its distributed feedback is able to oscillate at a single wavelength Wavelength selectivity is effected by a diffraction grating formed in the element. Optical fiber communication systems using as a light source such a single wavelength oscillation semiconductor laser has the advantage that despite wavelength dispersion caused by the optical fibers the optical signal transmitted through the optical fibers will not be subjected to distortion over long-distance transmission. DFB lasers therefore are promising as a light source for long-haul optical fiber communication.
The conventional DFB laser has a band with no oscillation mode, which is called stopband, in the vicinity of the Bragg wavelength determined by the period of the diffraction grating, and two oscillatory modes existing on both sides of the stopband. Owing to this, all elements do not oscillate at a single wavelength and some oscillate in two axial modes. For solving this problem, it has been proposed in, a published paper, Electronics Letters Vol. 20 (No. 8) pp. 326-327, issued on Apr. 12, 1984, to use a .lambda./4-shifted DFB laser which has a diffraction grating structure with a .lambda./4-shift region at the center of the DFB laser, the character .lambda. being the wavelength of light propagating therein. This laser can have a stable single oscillation at the Bragg wavelength because the threshold gain of the main mode (corresponding to the Bragg wavelength mode) oscillation is remarkably low compared with that of a side mode oscillation. The paper also discloses the theoretical study on such lasers either with an unreflective facet at each end or with an unreflective facet at one end and a 30% reflectivity cleavage facet at the other end, leading to the conclusion that the .lambda./4-shift region may be ideally located at the center of the element.
The inventor's study revealed that the above stated conclusion; that is the .lambda./4-shift region should be located at the center of the elements is not always so: though it is appropriate when the end facets are unreflective or equally reflective. However, the inventor determined that the optimum location of the .lambda./4-shift region should be deviated from the center of the element when the end facets have different reflectivities.
N. Eda et al discloses a theoretical study on the increase in efficiency of a .lambda./4-shifted DFB laser diode upon the introduction therein of structural asymmetry. (See the paper entitled "High Efficiency Active Distributed-Reflector Lasers of Asymmetric Structure", NATIONAL CONFERENCE RECORD, 1984 OPTICAL AND RADIO WAVE ELECTRONICS, PART 2, PAPER No. 271, October 1984, THE INSTITUTE OF ELECTRONICS AND COMMUNICATION ENGINEERS OF JAPAN.) Such asymmetry introduces a difference in lengths between the left and right regions with respect to the .lambda./4-shift region and a difference in coupling coefficients of light with the grating between the left and right regions. However, in the paper, the ends of the DFB laser are assumed to be reflection-free and no suggestion is given as to the location of the phase shift region for establishing stable single wavelength oscillation when the ends have different reflectivities.
Further it is known concerning the DFB laser that to suppress Fabry-Perot mode oscillation, which is generally unnecessary for the laser, and to increase the efficiency of the light taken from the front facet, the front facet of the element is frequently coated with an antireflection coating, as exemplified in the report, Electronics Letters Vol. 20 (No. 6), pp 233-235, issued on Mar. 15, 1984, in which is found no description of where a .lambda./4-shift region should be located.